It is known to provide a fuel injector with a control valve arrangement which is arranged to control movement of a fuel injector valve needle relative to a seating so as to control the delivery of fuel from the injector. Movement of the valve needle away from the seating permits fuel to flow from a delivery chamber through an outlet of the injector into the engine cylinder or other combustion space.
The control valve arrangement includes a control valve member which is movable between a first position, in which fuel under high pressure is able to flow into the control chamber, and a second position in which the control chamber communicates with a low pressure fuel reservoir. A surface associated with the valve needle is exposed to fuel pressure within the control chamber such that the pressure of fuel within the control chamber applies a force to the valve needle to urge the valve needle against its seating.
In order to commence injection, the valve arrangement is actuated such that the control valve member is moved into its second position, thereby causing fuel pressure within the control chamber to be reduced. The force urging the valve needle against its seating is therefore reduced and fuel pressure within the delivery chamber serves to lift the valve needle away from its seating to permit fuel to flow through the injector outlet. In order to terminate injection, the valve arrangement is actuated such that the control valve member is moved into its first position, thereby permitting fuel under high pressure to flow into the control chamber. The force acting on the valve needle due to fuel pressure within the control chamber is therefore increased, causing the valve needle to be urged against its seating to terminate injection.
Problems can occur in such arrangements as, when the control valve member is moving between its first and second positions, significant unbalanced hydraulic forces act on the control valve member. In particular, when it is desired to terminate injection, unbalanced forces acting on the control valve member serve to resist movement of the control valve member from its second position to its first position. The unbalanced forces acting on the control valve member therefore cause the control valve member to ‘hover’ between its first and second positions such that the re-establishment of high pressure fuel within the control chamber is either delayed or prevented. As a result, the valve needle of the injector may ‘dither’ between injecting and non-injecting positions, and this has a detrimental effect on injector performance.
It is an object of the present invention to provide a control valve arrangement which removes or alleviates the aforementioned disadvantage.